A solid-state type X-ray detector of X-ray CT apparatus is becoming dominant. Since the sensitivity of the X-ray detector has improved, effects of after-images resulting from an afterglow of scintillators have become notable, thus decreasing the time resolution and the quality of pictures. Specifically, such X-ray detectors include scintillators that generate fluorescent when receiving X-rays, detecting the X-rays by detecting the fluorescent with photodetecting means such as a photodiode. The fluorescent of the scintillators, however, does not stop immediately even after termination of the X-ray irradiation, to attenuate together with a delay in response, causing afterglow. Such afterglow is mixed in projection data to be sampled, causing after-images (artifact). Several methods are presented to correct the afterglow causing the after-images.
One of the methods is a method of correcting the shortest time-constant component at any time (refer to Japanese Examined Patent Publication No. 7-090024). Another method is a correction method using linear transformation of detector output (refer to Japanese Examined Patent Publication No. 7-102211). Another method is a correction method in correspondence with plural time constants at any time (refer to Japanese Unexamined Patent Publication No. 6-90945). Another method is a correction method using convolution integration (refer to Japanese Unexamined Patent Publication No. 6-343629).
The method disclosed in Japanese Unexamined Patent Publication No. 6-90945 is to acquire the time constant of the longest impulse response time and its component using a device including a radiation detector having an exponential impulse response determined by plural different time constants, and after eliminating the component, to acquire the next longest time constant and its component ratio and eliminate the component, wherein the process is repeated until influence of afterglow components is lost. It is described that the data of the time constants and their components are obtained in a factory by operating a CT apparatus with no object of tomogram present in an imaging opening. In other words, the impulse response data is collected only before shipping products from a factory.
However, since the characteristics of the X-ray detector of detecting afterglow and so on vary depending on environments including temperature during photographing of an object and secular change, the aforesaid methods are possibly unable to sufficiently eliminate afterglow components.
Such X-ray CT apparatus are required to accurately extract and eliminate afterglow components contained in projection data for further reducing effects of afterglow.